A plant such as fruit, vegetable, and cereal generally includes sugar. An amount of sugar in the plant is represented by a sugar content. The sugar content affects a commercial value of plant depending on a type of the plant. Therefore, in recent years, technical developments for increasing a sugar content of a plant have been carried out.
For example, tomatoes of high sugar content are produced mainly by soil culture. Further, a technique for producing tomatoes of high sugar content by nutrient solution culture has been suggested (Patent Literature 1).
It is known that a substance for regulating an oxidation-reduction state of a cell, such as glutathione, can function as a differentiation control agent for a cell or an organ (Patent Literature 2). Further, it is known that glutathione can function as a plant growth control auxiliary agent (Patent Literature 3).